An ion exchange membrane is widely used as a membrane for cells such as solid polymer type fuel cells, redox flow cells and zinc-bromine cells and as a membrane for dialysis. A solid polymer type fuel cell comprising an ion exchange membrane as an electrolyte is one of generation systems which take out chemical energy generated from a reaction between continuously supplied fuel and an oxidizing agent as electric power. The generation system is clean and highly efficient. Importance of this generation system has been recently grown in the fields of automobiles, home electric appliances and portable devices because of its operability at a low temperature and possibility of size reduction.
The solid polymer type fuel cell is generally constituted of a membrane made of a proton-conductive solid polymer acting as an electrolyte and a fuel chamber and an oxidizing-agent chamber separated by the membrane. A gas diffusion electrode carrying a catalyst is joined to both sides of the membrane. Fuel such as hydrogen gas or methanol is supplied into a chamber (fuel chamber) having one of the gas diffusion electrodes whereas oxygen or an oxygen-containing gas such as air is supplied as an oxidizing agent into a chamber (oxidizing-agent chamber) having the other gas diffusion electrode. An external load circuit is connected between the gas diffusion electrodes to supply power to the external load circuit, which allows the system to function as a fuel cell.
Among proton-conductive type fuel cells, a direct methanol type fuel cell directly using methanol as a fuel is easily handled because the fuel used is liquid and the fuel is inexpensive. Thus, a direct methanol type fuel cell is expected to be used as a relatively lower output power source for portable devices.
The basic structure of a conventional direct methanol type fuel cell is shown in FIG. 1. In FIGS. 1, 1a, 1b are cell partition walls which are disposed facing each other; 2 is a fuel flow hole as a groove formed in the inner surface of the partition wall 1a; 3 is an oxidizing gas flow hole as a groove formed in the inner surface of the partition wall 1b; 6 is a solid polymer electrolyte membrane having a fuel-chamber side diffusion electrode 4 in one side and an oxidizing-agent chamber side gas diffusion electrode 5 in the other side. The fuel chamber 7 and the oxidizing-agent chamber 8 are electrically insulated from each other by the solid polymer electrolyte membrane 6.
In this direct methanol type fuel cell, methanol is supplied to the fuel chamber 7. The supplied methanol produces protons (hydrogen ions) and electrons by the action of the fuel-chamber side diffusion electrode 4. The protons generated move through the solid polymer electrolyte 6 to the oxidizing-agent chamber 8 in the other side, where the protons are reacted with oxygen in the air or oxygen gas to produce water. On the other hand, the electrons produced on the fuel-chamber side diffusion electrode 4 move through the external load circuit (not shown) to the oxidizing-agent chamber side gas diffusion electrode (5), during which electric energy is applied to the load circuit.
In a direct methanol type fuel cell having such a structure, a cation exchange membrane is generally used as a membrane. The cation exchange membrane must meet the requirements for properties such as a low electric resistance, higher physical strength and lower permeability of methanol used as a fuel.
For example, when a cation exchange membrane exhibiting higher methanol permeability is used as a membrane for a fuel cell, diffusion of methanol in the fuel chamber into the oxidizing-agent chamber side cannot be completely prevented, leading to an insufficient battery output.
A perfluorocarbon sulfonic acid membrane, typically Nafion™, has been frequently used as a cation exchange membrane used as a membrane for a direct methanol type fuel cell. This membrane exhibits excellent chemical stability. Its physical strength is not adequately high to form a thin membrane, so that electric resistance of the membrane cannot be reduced.
In addition, since methanol is used as a fuel, the perfluorocarbon sulfonic acid film is deformed due to considerable expansion with methanol. Furthermore, such membrane expansion further accelerates diffusion of methanol into the oxidizing agent chamber side. Furthermore, a perfluorocarbon sulfonic acid film is expensive.
For solving the above problems, there have been investigated, in place of a fluorine-containing polymer such as a perfluorocarbon sulfonic acid, various cation-exchange membranes comprising a hydrocarbon polymer as a basic material. For example, Japanese Patent Applications Laid-open No. 2001-135328 and 1999-310649 have proposed a technique that by a particular method, a polyolefin or fluoro-resin porous membrane as a base member is impregnated with a monomer having a functional group to which a cation-exchange group can be introduce, the impregnated monomer is polymerized and then a cation-exchange group is introduced to the resulting polymer. These references have described that the process can provide a cation-exchange membrane having a lower electric resistance and hydrogen-gas permeability.
However, even in a cation-exchange membrane prepared by such a method, methanol permeability cannot be adequately minimized when it is used as a membrane for a direct methanol type fuel cell. As a result, there still remains the problem of reduction in battery performance due to diffusion of methanol from the fuel chamber side to the oxidizing-agent chamber side. Furthermore, when changing a membrane composition for minimizing methanol permeability, an electric resistance of the membrane increases, leading to reduction in a battery output.
We have proposed an ion exchange membrane having an additional layer consisting of an inorganic filler and ion exchange membrane on a porous membrane as a base member, as a membrane for a fuel cell with lower methanol permeability (Japanese Patent Application Laid-open No. 2004-217921). The ion exchange membrane cannot, however, adequately minimize methanol permeability.
In addition, there have been described that a so-called polyion complex membrane as a mixture of a polymer having an intramolecular acidic group such as a sulfonated polyether ether ketone and a polymer having an intramolecular basic group such as polybenzimidazole is used as a membrane for a direct methanol type fuel cell (Japanese Patent Application Laid-open No. 2003-535940). This polyion complex membrane has been conventionally investigated for the use in a membrane for a fuel cell in which hydrogen gas is used as a fuel. The reference has described that the membrane exhibits higher proton conductivity, high-temperature stability and lower methanol permeability.
Our investigation has confirmed that the membrane is more effective to some extent in improving stability against swelling with methanol and in reducing methanol permeability in comparison with a membrane made of each polymer alone. However, it has been demonstrated that since the membrane is a so-called non-crosslinked membrane without a covalent-bond bridge, it is still unsatisfactory in minimization of methanol permeability.
The content of an acidic or basic group in a polyion complex membrane might be reduced to minimize methanol permeability. This method, however, lead to reduction in proton conductivity in the polyion complex membrane.
Furthermore, in the preparation of the above polyion complex membrane, the acidic-group containing polymer and the basic-group containing polymer must be mixed, during which precipitation occurs. For avoiding the problem in the above method, a mixed solution of these polymers is first made basic before membrane forming and then the resulting membrane is treated with an acid. The process is, however, troublesome.
It is also known that a crosslinking type cation-exchange resin such as a sulfonated styrene-divinylbenzene resin is impregnated with a polymer having an anion-exchange group such as polybenzimidazole to form a polyion complex membrane (Japanese Application Laid-open No. 2001-167775). However, in such a membrane, the higher a crosslinking degree in the crosslinking type cation-exchange resin is, the less freedom of movement of the impregnated polymer having an anion-exchange group in the membrane is, so that the amount of ion complexes formed is reduced and thus the membrane cannot be sufficiently effective.
Furthermore, a polybenzimidazole type polymer having a benzimidazole structure in a motif in a principal chain, whose principal chains can be crosslinked has been used as an ion exchange resin which is to be a material for a membrane for a fuel cell because of, for example, its excellent heat resistance. For further improving its heat resistance, it has been proposed that a sulfonic acid group is introduced to a benzene ring coexisting in the principal chain (Japanese Patent Application Laid-open No. 2004-131533). In such a polybenzimidazole type polymer, an acidic group such as a sulfonic acid group can be introduced via a hydrocarbon group to an NH group in an imidazole ring in its principal chain, and thus it has been proposed that a membrane for a fuel cell is produced using an ion exchange resin thus prepared by introducing an acidic group (Japanese Patent Application Laid-open No. 2002-535467).
It can be assumed that in a polybenzimidazole type polymer to which the above acidic group has been introduced, the introduced acidic group and the imidazole ring in the principal chain would form an ion complex to some extent. However, in such a structure where an imidazole ring is contained in a principal chain, freedom in a reaction is too low to form the ion complex in a high yield. Even a membrane for a fuel cell formed by such a procedure cannot, therefore, reduce methanol permeability to a sufficient level.
As described above, there are no conventional cation-exchange membranes used for a membrane in a direct methanol type fuel cell exhibiting both low methanol permeability and high proton conductivity.